We have established that oncoproteins which function as dysregulated components of mitogenic signaling pathways cause marked feedback inhibition of physiologic signaling. Much of our recent work has focused on understanding the implications of this phenomenon. Insensitivity to feedback inhibition of receptor activation of wild type RAS is a common property of oncoproteins that activated ERK signaling that is required for them to elevate ERK output. Second, elevated pathway output includes elevated feedback inhibition of physiologic signaling pathways. This is a major determinant of the so- called oncoprotein dependence of transformed cells. Third, feedback-dependent oncoprotein pathway dependence reduces the robustness of the cell and creates a selection for mutations that activate feedback-Inhibited pathways and restores robustness. This accounts for some of the secondary driver mutations identified in tumors. Finally, inhibitors of oncoprotein-activated signaling have significant antitumor activity, but also relieve feedback inhibition of physiologic mitogenic signaling pathways and cause their reactivation. This attenuates their antitumor activity and creates a logic for inhibiting key reactivated pathways in tumors exposed to inhibitors of oncoproteins. This strategy has had some early clinical success and has become a paradigm for the development of rational combination therapies. Despite these insights, we still know only few of the details of oncoprotein-induced feedback and its relief by targeted inhibitors. We do know that these details vary as a function of tumor lineage and which pathway component is mutationally activated. Moreover, the effects of relieving feedback also vary depending on which node of the pathway is pharmacologically inhibited. We now plan to comprehensively study feedback and its relief by nodal inhibitors, focusing on a few tumors and using both methodologies biased by previous knowledge of normal signaling and unbiased shRNA screens. We utilize selective inhibitors of PI3K, AKT, mTOR, RAF, MEK, ERK, and a novel allele-specific inhibitor of RAS and study both short- and long-term adaptation, to determine whether some of the effects of the latter are due to epigenetic regulation. The goal is to develop new effective combination therapies based on these data and on in vivo studies to determine dose schedules that optimize induction of cell death.